Real-time monitoring of an optical network system is important for controlling and improving the performance of the optical system. One known practice for real-time monitoring involves the use of a local memory, controllers, and look-up-table (LUT) circuits to adjust and to set the main system parameters. The main disadvantage of this method, however, is that this method requires a complicated initial calibration for the optical network channel before using the system.
Similar methods of self-adjustment (alternatively referred to herein as “adaptive adjustment”) are also used in practice to adjust the parameters of optical systems. In the system described in U.S. Pat. No. 5,929,982, for example, the adaptive adjustment of APD gain is achieved by adaptive optimization of the receiver signal-to-noise ratio. Specifically, for monitoring purposes, the noise level over some threshold is measured and is used for adaptive setting. However, a drawback of using an adaptive system to adjust the APD gain based on the measurement of noise over some threshold and calculating the signal to noise ratio is that, although this method may give adequate results when the received optical signal has a good quality with a clearly open eye diagram, when the quality of the eye diagram is not good, e.g., when there is optical dispersion in the fiber or there is insufficient signal time jitter, this method may not provide clear results and also may not allow for troubleshooting of faults in the signal link.